Newsgroups: rec.audio.tech
Subject: Re: Sound out from computer to receiver - hum is heard!
References: 
"LarryLOOK"  writes:

> This is a repost with some new developements.  Hope someone has a 
> suggestion.  I'm trying to run sound out from a PC and laptop to my 
> receiver.  I'm using the 1/8 inch stereo cable from line out from computer 
> (marked with light green) with Y-adapter to two rca cables which are plugged 
> into receiver.  The volume is low so receiver volume needs to be turned up 
> pretty high, unfortunately - even with computer volume turned to max.

Sounds strange.... 
In all computers I have connected to audio system, and that's tens of them, 
there has not been a problem that the sound from the PC would be 
very low when the sound card settings are right and connections are right. 

In al the cases there has been problems the case has been one of the 
following:
- wrong soundcard settings (main volume or indidual sound source 
  volume setting on computer sound card mixer too low)
- some cable plugged to wrong connector (cable should be from 
  speaker/line/headphone outlet to amplifier line in)
- wrong amplifier settings (you are accentally selected 
  different input than you have sound coming in, and what you just 
  hear is just crosstalk from input to another inside baddly designed 
  crappy amplifier)
- bad connection on connectors on PC or amplifier end
- broken cable
 
> When I do this with 2 PC's and laptop there's a bad hum with all three! 
> However I can get rid of hum by unplugging laptop from AC and using it's 
> internal battery.  

Definately sounds like a ground loop problem to me.

> So I tried buying a AC line interference filter ($20 at 
> radio shack) but it won't prevent hum.  

AC line filters do not help in solving ground loops. 

> So what is the best solution?  

I have written on this at 
http://www.epanorama.net/documents/groundloop/index.html

The proven solutions that work (can use any one of them):
- use computer only with it's batteries disconnected from mains
- use an audio isolator on the audio wire
- power the computer through a safety isolation transformer

Sometimes what can help:
- disconnecting the cable TV connection / antenna cable going to wall

> Do I 
> try to ground the 3 computers better with a wire from ground to computer 
> casing or is there some other solution?  

I do not recommend this. 
Usually this kind of extra grounding does not solve the problem 
and can cause problems.

> I can't believe it's so difficult 
> to get decent sound out of a computer to a receiver - but I guess it is! 

Generally if you follow this tips you get proper sound:
1. Plug your computer and amplifier to same electrical outlet 
   (for example to same extension cord)
2. Do not plug the amplifier to anything else
3. Use a workign cable wired correctly 
4. Set the computer sound cards settings right

Now you shoudl have a crystal clear sound without any problems. 
This tips works every time. 


Other thing that works alsow well:
1. Let your audio system to be as it is
2. Put the 3.5 mm to RCA cable to computer soundcard 
3. Connect the RCa end to amplifier line input through 
   "RCA audio isolator" device
4. Set the computer sound cards settings right

This will give you well you noise free sound well. 
The "RCA audio isolator" device can have some effect 
on your sound quality, how much depends how good isolator 
you use and somewhat on the sound source you have. 



-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/